This invention relates generally to catalytic materials and membranes that are used in the chemical and petroleum refining industry, and more specifically to high-flow catalytic zeolite membranes and methods of synthesis.
Zeolites comprise microporous inorganic aluminosilicates with pore diameters in the 3–7 Å range. They are commonly used in bulk form for gas separations, as catalysts, and for dehydration.
The commercial market for industrial membranes is growing rapidly and is expected to reach $3.5 billion by 2005. Zeolitic membranes figure prominently in this rapidly developing area. Development of new methods of zeolite synthesis and new methods of membrane synthesis will greatly expand the variety of organic and inorganic molecular separations possible. Substitution of zeolites in membrane form, as opposed to a loose powder, would allow these industrial processes to operate using continuous flow reactors, as well as eliminate the use of binders, thereby substantially reducing energy consumption and overall operating costs.
A particularly useful zeolite, ZSM-22, is currently used as a catalyst in many industrially important applications, including: (a) olefin isomerization (specifically production of MTBE, among others); (b) catalytic dewaxing (refining of heavy oils); (c) toluene alkylation (for production of p-xylene); (d) isomerization of aromatic compounds (for production of p-xylene); and (e) production of jet fuels. All of these industrial processes currently utilize the zeolite in bulk form (e.g., powders, granules and pellets held together with binder phases) in batch type reactors because ZSM-22 is not commercially available in membrane form.
Historically, zeolites with the high-flow, open TON crystal structure were synthesized first as Theta-1 [1], KZ-2 [2], Nu-10 [3], and ZSM-22 [4] via hydrothermal methods. These were later determined to have identical crystal structures though varied morphology [5]. This family of zeolites is characterized by high aspect ratio crystals with uni-dimensional pores formed by 10-member rings. The shape selective and catalytic properties of TON zeolites are widely known, though there is some disagreement as to the source of these properties [6–10]. Platinum doping [11], ion exchange [12], framework metal substitution [13], and silicon deposition [14] are all methods shown to be useful in enhancing desirable properties for industrial applications. In particular, ZSM-22 has the ability to isomerize 1-butene to 2-butene, which is a precursor in the manufacture of methyl tert-butyl ether [15].
Zeolites are typically synthesized by hydrothermal or solvothermal methods; ZSM-22 solely by hydrothermal methods [1–4]. Recently, however, Vapor Phase Transport (VPT) and Dry Gel Conversion (DGC) have been used to synthesize a variety of zeolites and zeolite membranes [16–22]. VPT refers to conversion of a dry, amorphous precursor into a fully crystalline material via contact with a vapor phase organic-water mixture, while DGC refers to conversion of a dry, amorphous precursor into a fully crystalline material via contact with only vapor phase water.
One difference between the vapor phase method versus traditional hydrothermal methods is that rather than dissolving or dispersing all the ingredients into a solution (i.e. into excess solvent), the ingredients, in proper proportion are mixed together and gelled into a non-crystalline, solid, precursor. A standard definition of gelation is forming a continuous, 3-dimensional interconnected (chemically bonded) network throughout the entire solution—the solution is transformed from a liquid to a solid without significant loss of volume or solvent. Such gels typically contain a lot of solvent, alcohols, water, etc. Gels are not really ‘solids’, but super viscous liquids—in the same manner as glass is a liquid—because they contain no long range structural order.
In order to crystallize this material, energy in the form of heat, and some additional organics are added (in the vapor phase). Some water may also be necessary to allow bond breakage, without which molecular rearrangement (i.e. crystallization) could not take place. The organics help to form nucleation sites (together with the inorganic gel) from which the crystal grows. A particular chemistry gel, i.e. elements in the correct range of proportions, as well as an adequate—but often not excessive—amount of vapor phase organic & water will grow crystals.
The type of vapor phase organic(s), the ratio of water to organic, and water/organic to solid, the type of chemical elements, the ratio of chemical elements, the degree of association between the chemical elements (largely determined/fixed at the point of gelation by the gelation method but also a factor of temperature and aging of gel), amount and type of liquid solvent used to form the pre-gel solution, the type and form of the chemicals used in the pre-gel solution—all help determine which crystal phase will nucleate and grow.
In VPT and DGC the outcome of the crystallization process can be controlled to a much greater degree because the precursor chemicals in the gel are “frozen”, whereas in hydrothermal synthesis all of the ingredients are just dumped into a solution and heated—there is much less control, or no control at all, of the factors listed above.
A membrane form of ZSM-22 could have different properties, such as: (1) a robust structure due to the crystal intergrowth, allowing it to have sufficient strength to be entirely un-supported; (2) a ZSM-22 membrane made up of 100% pure crystalline zeolites without any binder phase would have no impurities or additional components that could possibly contaminate or interfere with processing of a hydrocarbon stream; (3) a ZSM-22 membrane could be high-flow, where high volumes (high Weight Hourly Space Velocities) of feed stock may be passed through the membrane past the crystals that have a very high surface area for catalysis without fouling or build-up of back-pressure, which would be an extremely important property for industrial membrane processes; and (4) ZSM-22 membranes could be fabricated and manufactured in a variety of sizes, thicknesses, shapes, and non-planar curvatures that could be tailored to meet a wide variety of specific needs. Synthesis of ZSM-22 membranes using VPT and/or DGC methods could reduce manufacturing costs since there would be no excess or wasted materials, and since there would be no need to add binders, which adds additional process steps.
A need remains, therefore, for a method of synthesizing un-supported, high-flow ZSM-22 zeolite membranes that do not have a binder phase. Against this background, the present invention was developed.